Measurement of fluid flow rates

ABSTRACT

Apparatus and a method for measuring the flow rate of a fluid in which a beam of ultrasonic, electromagnetic, optical or other radiant energy is transmitted across the flow at each of two positions spaced apart in the direction of flow, the noise amplitude, frequency or phase modulation on each beam due to disturbances in the flow is detected, and the two resultant signals are cross-correlated to determine the time delay therebetween producing maximum correlation, i.e. the mean fluid transport time between the beams, and thus the mean fluid flow rate.

Unlted States Patent 11 1 1111 3,762,221 Coulthard Oct. 2, 1973 [54]MEASUREMENT OF FLUID FLOW RATES 3,197,625 7/1965 Ratz 324/77 G X [76]inventor: John Coulthard, l Greencroft 3,588,699 6/1971 Pysnlk 73/194 ERedcar, England OTHER PUBLICATIONS Correlation in Action,Hewlett-Packard Journal [221 July 1971 (Nov. 1969), vol. 21, No 3, pp.17-20 [21] Appl. N0.: 159,864

Primary ExaminerCharles A. Ruehl [30] Foreign Application Priority DataAtmmey Young & Thompson July 6, 1970 Great Britain 32,563/70 Jan. 19,1971 Great Britain 2,593/71 [57] ABSTRACT Apparatus and a method formeasurmg the flow rate of [521 1 CI. n E 235 a 11'! which a b63111 Ofultrasonic, electromagnetic, 511 1111. C1. (1011 1/00 "Ptical radiantenergy is transmitted the 581 Field of Search 73/194 E 194 A eachWsitions SP1ced aPart the 73/194 F, 194 B, 204; 324/77 G;235/181, 151.34

tion of flow, the noise amplitude, frequency or phase modulation on eachbeam due to disturbances in the flow is detected, and the two resultantsignals are crosscorrelated to determine the time delay therebetweenproducing maximum correlation, i.e. the mean fluid transport timebetween the beams, and thus the mean fluid flow rate.

34 Claims, 9 Drawing Figures v V 1 E 2:

R 1 RI AM 3 AM S/GAAL CORRELATOR PATENTEU BET 2 873 SHEET 3 UF 5 T r 1tA 1/ A z PM we 20/ FREQUENCY- FREQUENCY 20/ 70- VOLT/1G5 TO-VOLTAGfCONVERTER cowaxrsn xlt y(t) r"- 5 SIGNAL CORRELATOR PHASE-TO- IPHASE-7'0- VOLTAGE VOLTAGE cowmmz CONVERTER 2 |x(t) WU] SIGNAL 15 [L706CORRELATOR/ PATENTED OCT 2 I975 SHEET 5 BF 5 PHASE-TO VOLTAGE CONVERTERMEASUREMENT OF FLUID FLOW RATES This invention relates to themeasurement of fluid flow rates.

SUMMARY OF THE INVENTION According to one aspect of the presentinvention, a method of measuring the rate of flow of a fluid comprisesthe steps of transmitting a beam of radiant energy into the fluid ateach of two positions spaced apart in the direction of flow of thefluid, receiving the two beams and detecting the noise modulation oneach beam due to disturbances in the fluid flow profile,cross-correlating the two detected noise signals, and determining thevalue of correlation delay producing maximum correlation between the twonoise signals.

The value of the correlation delay producing maximum correlationprovides a measure of the time taken for disturbances in the flow, whichare transported at the mean velocity of the flow, to pass from oneposition to the other, and thus, if the spacing between the twopositions is divided by the value of the correlation delay producingmaximum correlation, the mean fluid flow rate is obtained.

It will be apparent that the invention is particularly applicable tomeasuring the velocity of a highly turbulent flow, as in such a case thedisturbances in the flow will be large. However, provided the detectionapparatus is sufficiently sensitive, the method may be used formeasuring the velocity of a flow which borders on the purely laminar. Inthe event that the flow is substantially non-turbulent, disturbances maybe deliberately induced in the fluid to render the method of theinvention more effective.

The two noise signals can be obtained by detecting amplitude, frequencyor phase modulation of the beams; various different embodiments of theinvention disclosed hereinafter show how these different detectiontechniques may be employed.

The method may be used to measure the flow rate of a gas, a liquid, or aslurry. Liquids having a large number of scattering centres, e.g. bloodor milk, are particularly suited to this technique of measurement, asscattering of the radiated beams is produced and hence the noise signalsare strengthened.

In a number of different embodiments of the invention described indetail hereinafter, the two beams of radiated energy lie in the sameplane, which also includes the direction of flow, so that the methoddetermines the correlation between the flow profile in said plane at twopositions spaced apart along the direction of flow. Alternatively, thetwo beams may lie in respective different planes each of which includesthe direction of flow, so that the flow is only correlated in and nearthe streamline passing through both beams. For example, the two beamsmay lie in mutually perpendicular planes and may both pass through thecentre of a duct in which the flowing fluid is contained. This lattermethod is particularly applicable to measuring the flow rates of liquidslike blood, due to reflection of the radiated energy from thecorpuscles, and is notably useful in surgical applications, as bloodflow in a patient can be monitored by transducers clamped to the outsideof the patients body.

A number of alternative type of radiant energy may be employed.Ultrasonic radiation and electromagnetic radiation may be employed, andoptical radiation such as laser propagated light may also be used.

In accordance with another aspect of the invention, apparatus formeasuring the rate of flow of a fluid comprises means for transmitting abeam of radiant energy into the fluid at each of two positions spacedapart in the direction of the flow of the fluid, means for receiving thetwo beams and detecting the noise modulation on each due to disturbancesin the fluid flow profile, and a signal correlator for cross-correlatingthe two detected noise signals.

The apparatus may also include means for determining the value ofcorrelation delay producing maximum correlation between the two signals,although this function may instead be performed by an operator.

In embodiments of the invention described in more detail hereinafter, ateach of the two positions, an oscillator is connected to a transmittingtransducer on one side of a duct carrying the flow, and a receivingtransducer is located on the other side of the duct. The electricaloutput signal from each receiving transducer is passed to an amplitudemodulation (AM), frequency modulation (FM) or phase modulation detectorto detect the noise modulation thereon. The two resultant detected noisesignals arethen fed to a signal correlator to determine the delaybetween them producing maximum correlation. When phase modulationdetectors are used the input of the associated transmitting transduceris connected to each detector to enable the modulation of the phaseshifts across the beams to be detected.

In a development of the above embodiments, positive feedback isintroduced between each receiving transducer and its associatedtransmitting transducer, whereby no oscillator is needed, the transducerfeedback loop self-oscillating at a frequency largely determined by theresonant frequency of the transducers. Such an arrangement obviates anyneed to continually adjust the frequency used to the resonant frequencyof the transducers, to obtain optimum performance, consequent to theresonant frequencies changing due to the effects of temperature,degradation, ageing of the transducers, and so forth.

In another embodiment, the transducers associated with both beams areconnected to form a single feedback loop including both beams.

When gas flow is to be measured, the apparatus may be so arranged that astanding wave pattern is set up in the fluid, so that disturbance of thepattern will increase the amplitude of the noise signals.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of theinvention may be obtained from a consideration of the followingdescription of a number of illustrative embodiments thereof which aregiven by way of example, having reference to the accompanying drawings,in which:

FIG. 1 shows schematically a first embodiment of apparatus according tothe invention, in which amplitude modulation of each of two ultrasonicbeams propagated across a duct shown in longitudinal section providesthe two noise signals;

FIG. 1A is a partial view of a variation of the embodiment of FIG. 1 inwhich the duct is shown in axial rather than longitudinal section;

FIG. 2 shows the variation in magnitude and phase of the signal receivedwith the diameter of a duct in'which a gas is flowing, a standing wavepattern being set up in the duct;

FIG. 3 shows sehcmatically a second embodiment of the invention in whichthe oscillators of FIG. 1 are replaced by regenerative feedback loops;

FIG. 4 shows schematically another embodiment in which the oscillatorsof FIG. 1 are replaced by a single regenerative feedback loop includingboth beams;

FIG. 5 shows schematically another embodiment in which frequencymodulation of each of two beams is detected;

FIG. 6 shows schematically another embodiment in which modulation of thephase shift across each beam due to disturbances in the profile of thefluid flow is detected;

FIG. 7 shows in block diagram form a phase-tovoltage converter for usein the apparatus shown in FIG. 6; and

FIG. 8 shows schematically, for one beam only, a variation of theapparatus shown in FIG. 6 in which the beam is passed through the ducttwice to reduce errors caused by distortion of the velocity profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment illustratedin FIG. 1, oscillators l0, 10 power electrical-toultrasonic transmittingtransducers T, T which are mounted on the outside of a duct 1 l and arearranged to transmit beams of ultrasonic energy generally at rightangles to a fluid flowing in the duct. The two beams lie in a commonplane which includes the direction of flow. The frequencies used arepreferably in the order of38-40 kHz for gases, and in the order of ZMHzfor liquids, but may vary outside these values in dependence on thefluid under test.

The beams are amplitude modulated by noise comprising disturbances inthe fluid and the noise modulated signals are received by receivingtransducers R, R which are mounted on the duct 11 opposite thetransmittingtransducers T, T and which convert the modulated ultrasonicsignals to electrical signals. These latter signals are amplified byamplifier 12,. 12', and after detection by AM detectors l3, 13 the noisesignals are amplified by amplifiers 14, 14' to produce two signals x(t)and y(t) which are then passed to a signal correlator 15 to becross-correlated, the correlation delay for maximum correlation beingdetermined to provide a measure of the mean fluid flow rate.

If the apparatus is to be used in an application where the flow of fluidthrough the duct 11 borders on the purely laminar, means shown generallyat A may be provided upstream of the beams so as to interrupt or agitatethe flow and thereby to artificially induce disturbances therein.

In a variation of the FIG. 1 embodiment shown in FIG. 1A the beams liein respective different planes each of which includes the direction offlow. The planes are orthogonal and intersect in the centre of the ductwhereby the apparatus only correlates flow disturbances in and near thestreamline through the centre of the duct. As mentioned before,detecting flow disturbance correlation in this fashion is particularlyadvantageous for measuring the flow rates of liquids such as blood dueto reflection of the radiant energy from the corpuscles.

The acoustic reflection coefficient between gas and metal is almostunity, and therefore, for each transducer, in an arrangement as shown inFIG. 1 used for measuring gas flow in a duct having internal walls ofmetal, the acoustic energy transmitted from one wall into the duct isreflected back from the opposite wall and transmitted back to the firstwall where further reflection occurs, this multiple reflection settingup a complex acoustic standing wave pattern.

Thus, for a duct which is circular in cross-section, the magnitude ofthe voltage output v, from each receiver transducer R, R is givenapproximately by the equation v,.= v [(sinh aD cos kD -j cosh aD sinkD)/(c0sh 2aD cos 2kD) and the phase difference between v and v is givenby the equation 0 tan [cosh aD sin kDlsinh aD cos kD] where D thediameter of the duct,

or the attenuation coefficient,

k the wave number defined as 21r/k,

v the voltage when D O, and

)t the wavelength of the radiation used.

The magnitude and phase of each received voltage with respect to thetransmitted signals is shown in FIG. 2 where they are plotted againstthe duct diameter.

When a gas flows along the duct, disturbances in the gas causecompressions and rarefactions to move through the acoustic fieldpatterns set up, causing noise fluctuations in the standing wave systemand variations in the amplitude v, of the signals from the receivingtransducers. If the oscillator signals are constant in amplitude andfrequency, the r.m.s. values of the signals x(t) and y(t) vary accordingto the magnitude of the above equation for v,. It follows that themagnitudes of the signals x(z) and y(t) depend upon the length of theacoustic path and the degree of turbulence of the fluid flowingthereacross. If the number of wavelengths in each beam is not identical,a particular disturbance might cause one noise signal to increase andthe other to decrease, thereby providing a negative crosscorrelationpeak, the method of invention thus still being practicable.

The apparatus of FIG. 1 is desirably operated at frequencies which areas close to the resonant frequencies of the transducers as is possible,particularly if high-Q transducers are used, to avoid the loss of highfrequency information which would degrade the crosscorrelation curve andthereby reduce accuracy. Thus,

if the resonant frequencies of the transducers change, due to effectssuch as ageing, temperature change, the accumulation of dust on thediaphragms of the transducers, and so forth, the oscillator frequencieswill constantly require correction. The embodiment of the inventionillustrated in FIG. 3 has therefore been devised.

In this embodiment, where the same reference numerals are used forelements corresponding to those used in the embodiment illustrated inFIG. 1, oscillators are not provided. Instead, the amplifiers I2, 12'and l6, 16 form parts of self-oscillating electro-acoustic positivefeedback loops L and L which include the transducers and the acousticbeams, the parameters of the loop components being such that thetransducers ensure that the loops oscillate at frequencies largelydetermined by their own resonant frequencies. The frequencies thusautomatically change as the resonant frequencies of the transducerschange.

The amplitudes of the oscillations are determined by saturation of theoutput stages of the amplifiers 16, 16', the signals for detection beingpicked off at the outputs of the amplifiers 12, 12', before anydistortion due to saturation occurs. These signals are directlyproportional to the magnitude of the acoustic pressure at the receivingtransducers.

Movement of the transducers in directions perpendicular to the directionof flow causes the oscillation frequencies to vary about frequenciescentred on the resonant frequencies of the transducers in an accuratelypredictable manner. When the arrangement oscillates at the transducerresonant frequency conditions exist for obtaining satisfactorycross-correlation between the signals x(t) and y(t).

In tests upon apparatus according to the embodiment of FIG. 3,disturbances of the standing wave patterns set up in the duct providedclear cross-correlation curves and hence accurate measurement of gasflow at velocities ranging upwards from about 4 m/s when the twopositions were spaced apart by 25 cm. The system frequenciesautomatically adjusted in response to environmental changes to produceconsistently accurate flow measurements. It was also found that thespacing between the two positions could be increased as the flow rateincreased.

It has been found that certain forms of the invention, in certainapplications, are subject to inaccuracies due to phase shift between thetwo beams. It is believed that this inaccuracy is due to displacement ofthe crosscorrelation peak from the position that it would occupy werethere no such phase difference, resulting in the measured value of delayfor maximum crosscorrelation between the two beams differing slightlyfrom the mean disturbance transport time between the two beams. Itshould be understood that this inaccuracy is found in certainapplications only; for example, the apparatus shown in FIG. 1 providesgood results when used to measure the flow rate of a liquid, yetprovides less satisfactory results when used for measuring the flow rateof a gas. It is believed that the less satisfactory results experiencedwhen this apparatus is applied to measuring the flow rate of a gas maybe due to disturbance of the standing wave pattern set up in the gas.

The embodiment shown in FIG. 4 broadly resembles the apparatusillustrated in FIG. 3. In this case, however, the amplifiers 12, 12 and16, 16', together with the two beams, form a single self-oscillatingelectroacoustic positive feedback loop. Like the apparatus shown in FIG.3, the loop oscillates at a frequency largely determined by the resonantfrequencies of the transducers, the frequency thus automaticallychanging as the resonant frequencies of the transducers change. Theapparatus shown in FIG. 4 has the advantage that the above-mentionedinaccuracy caused by the effects of phase difference between the twobeams is reduced considerably as compared with the apparatus of FIG. 3.

It should be noted that no crosstalk occurs between the two modulatingnoise signals in FIG. 4 even though they are both incorporated in thesame feedback loop. This is because the amplifiers 16, 16' both havesaturated outputs whereby the modulation is removed.

It will be apparent that as well as the preferred feedback loopR-l2-l6T'-R-l2-l6'-T-R there are two possible subsidiary feedback loopscaused by propagation of unwanted beams between T and R and between Tand R. The apparatus must, of course, be arranged so that the preferredfeedback mode is at least predominant. To this end, the transmittingtransducers T, T' are relatively matched, and the receiving transducersR, R are mismatched both mutually and relative to the transmittingtransducers T, T.

The embodiment illustrated in FIG. 5 is also broadly similar to thatillustrated in FIG. 3. Here, however, instead of detecting the amplitudemodulation on the beams due to disturbances in the flow, the variationin frequency, i.e. the frequency modulation of the beams due to thedisturbances in the flow, is detected by the illustratedfrequency-to-voltage converters i.e. FM detectors 20, 20' to provide thesignals x(t) and y(t) for correlation.

It will be appreciated that the embodiment of FIG. 5 can be modified byexcluding the feedback loops and by using oscillators to power thetransmitting transducers as in the embodiment of FIG. 1.

Theernbodiment of the present invention illustrated in FIG. 6 broadlyresembles the apparatus shown in FIG. 1 in that two separate acousticbeams fed by respective oscillators 10, 10' are used. I-Iere, however,instead of detecting amplitude or frequency modulation on the beamscaused by disturbances in the fluid flow, the total phase shift acrosseach beam is measured by feeding the oscillator output and the receivertransducer output, for each beam, into a respective phaseto-voltageconverter 22, 22'. Due to the fact that a disturbance moving through thetwo beams causes the phase between the transmitter transducer input andthe receiver transducer output to change in a similar manner in eachbeam, there is no phase shift between the outputs signals of thephase-to-voltage converters 22, 22' and these two signals can becross-correlated giving a result for the transport time for disturbancesbetween the two beams which is not rendered inaccurate by theabove-mentioned phase shift effects.

A phase-to-voltage converter suitable for use in either of the twochannels in the apparatus of FIG. 6 is shown in FIG. 7 in block diagramform, together with the waveforms appearing at certain points therein.The signals from the output of the oscillator 10 (10) and from theoutput of the amplifier 12(12) are applied to first inputs of respectivegates 30 and 32. These signals are trains of pulses at the ultrasoniccarrier frequency which are constantly changing phase relatively to eachother due to turbulences in the flow. The outputs of the gates 30 and 32are connected to divider circuits 34 and 36. In this embodiment thesecircuits divide by eight although they may, in different applications,be arranged to divide by different figures or not be provided at all.The dividers are only necessary when the apparatus is to be used withducts having large diameters where they bring the phase shifts withinthe range of linearity of the converters.

The output signal of the divider 34, which has a period eight timesgreater than the input signal, is applied to a linear quarter perioddelay network 38 which gives rise to a ramp waveform at the outputthereof as illustrated. The ramp output of the linear quarter perioddelay network is connected to a level detector 40 which changes state atits output when the input voltage thereof exceeds a predetermined level.This output is connected to a half period monostable circuit 42 togenerate the waveform shown.

This waveform is applied to a first input of a further gate 44, theoutput of the divider 36 being applied to the other input of the gate.Due to the fact that the two input voltages to the gate 44 representlike waveforms displacedin time by a quarter period of the dividedwaveform and these vary in phase with respect to each other about thismean value as turbulent disturbances move through the beams, the outputvoltage of the gate 44 is a variable length pulse the length of which isa measure of said phase shift. Successive pulses from the outputof thegate 44 are fed to an integrator 46 which thus gives as its output asignal proportional to the mean phase shift across the beam, i.e. x(t)or y(t) as the case may be. It will be appreciated that if the flowvelocity is zero, the pulse width of the output waveform of the gate 44will be constant and thus the integrator output will have a constantd.c. level. When flow occurs, the pulse width varies producing thewaveform shown in the form of an 21.0. perturbation on a mean d.c.level. To obtain the signal x(t) or y(t) in a.c. form the output of theintegrator output is a.c. coupled.

Logic synchronising pulses obtained from a pulse width detector 48connected to theoutput of the gate 44 are applied to second inputs ofeach of the gates 30 and 32 and to reset terminals of the dividers 34and 36. These synchronising pulses are only supplied when the dividersare out of synchronism, as, for example, when the apparatus is firstswitched on. The pulse width detector 48 detects lack of synchronismbecause, if the dividers 34 and 36 are not triggered by the leading edgeof the same pulse of the ultrasonic carrier waveform, a false pulsewidth will be obtained, as will be apparent from a consideration of FIG.7.

FIG. 8 shows in block diagram form the apparatus associated with onebeam only in a variation of the apparatus illustrated in FIG. 6. In FIG.8, the duct 11 is viewed in radial section rather than in axial section.The beam of acoustic energy is propagated across the duct twice to givea more accurate measure of the average flow velocity, particularly ifthe velocity profile is distorted. It is first propagated bytransmitting transducer T1, is received by receiving transducer RI andpassed to a second transmitting transducer T2, and tinally picked-up byreceiving transducer R2. It will readily be seen that the apparatus canbe simply moditied for three or more passages of the beam across theduct if required to give an even more accurate reading.

It will readily be apparent, from a consideration of the foregoingdisclosure, that the invention may be carried out using electromagneticradiation instead of using ultrasonic energy. The only basic change isto replace the transducers T, T and R, R, which may be piezoelectricdevices, by appropriate means such as aerials for radiating electricalenergy into the duct and for receiving the radiation at the other side.The noise signals in this case would be due to modulation of the beamsdue to changes in the electrical properties of the flowing fluid causedby the disturbances, rather than to changes in pressure caused by thedisturbances.

It is also contemplated that an optical analogy of the ultrasonicembodiments can use changes in the optical properties of the movingfluid to modulate beams of light. Thus, the transducers T, T could bereplaced by alternating light sources, and the transducers R, R byphotoelectric detectors. In this connection, lasers could be used in anapparatus corresponding to that illustrated in FIG. 3, a pair of lasersbeing combined with a mirror arrangement to produce a self-oscillatin gfeedback arrangement analogous to the eIectro-acoustic feedback loops L,L.

. The present invention can be seen from the foregoing description toprovide a method of and an apparatus for measuring the flow rate of alltypes of fluids, giving results which are absolute and independent ofthe physical properties of the fluid. As there is no need to obstructthe fluid flow by mounting parts of the apparatus in the flow,dangerous, corrosive, high-pressure and other hostile fluids can safelybe monitored.

I claim:

1. Apparatus for measuring the rate of flow of a fluid, comprising meansfor transmitting a beam of radiant energy, into and across the fluidflow at each of two positions spaced apart in the direction of flow ofthe fluid, respective receiving means associated with each beam and eachpositioned to receive the as-sociated beam after passage across theflow, each receiving means producing a beam-representative carriersignal modulated by a noise signal caused by disturbances in the fluidflow profile, a respective detector connected to each receiving meansfor detecting said noise signals, and a signal correlator forcross-correlating the two detected noise signals whereby the value ofcorrelation delay producing maximum correlation between said two noisesignals can be ascertained.

2. Apparatus as claimed in claim 1, wherein the signal correlatorincludes means for determining the value of correlation delay producingmaximum correlation between said two noise signals.

3. Apparatus as claimed in claim 1, wherein said transmitting means isarranged to transmit the two beams in acommon plane which also includesthe direction of fluid flow.

4. Apparatus as claimed in claim 1, wherein the transmitting means isarranged to transmit the two beams in respective different planes eachof which includes the direction of fluid flow.

5. Apparatus as claimed in claim 4, where said two planes areorthogonal.

6. Apparatus as claimed in claim I, wherein said transmitting meansincludes a respective transmitting transducer for each beam and whereinsaid receiving and detecting means includes a respective receivingtransducer for each beam.

7. Apparatus as claimed in claim 6, wherein an oscillator is connectedto the input of each transmitting transducer.

8. Apparatus as claimed in claim 6, wherein the output of each receivingtransducer is connected by a feedback loop to the input of theassociated transmitting transducer, each feedback loop having sufficientgain to cause it to self-oscillate so as to produce the associated beam.

9. Apparatus as claimed in claim 8, wherein the parameters of thecomponents of each feedback loop are selected so that it oscillates at afrequency which is near or equal to the resonant frequencies of theassociated transmitting transducer and receiving transducer.

10. Apparatus as claimed in claim 6, wherein the output of the receivingtransducer associated with a first of the beams is connected to theinput of the transmitting transducer associated with the second of thebeams and the output of the receiving transducer associated with thesecond beam is connected to the transmitting transducer associated withthe first beam, whereby both beams are included in a single feedbackloop having sufficient gain to cause it to self-oscillate to producesaid beams.

11. Apparatus as claimed in claim 10, wherein the two transmittingtransducers are mutually matched and wherein the two receivingtransducers are mismatched both mutually and relative to thetransmitting transducers.

12. Apparatus as claimed in claim 6, wherein said receiving anddetecting means includes a respective AM detector connected to theoutput of each receiving transducer for detecting noise amplitudemodulation on the beams to provide said two noise signals.

13. Apparatus as claimed in claim 6, wherein said receiving anddetecting means includes a respective FM detector connected to theoutput of each receiving transducer for detecting noise frequencymodulation on the beams to provide said two noise signals.

14. Apparatus as claimed in claim 6, wherein said receiving anddetecting means includes, for each beam, a respective phase-to-voltageconverter having a first input connected to the input of the associatedtransmitting trans-ducer and a second input connected to the output ofthe associated receiving transducer, each converter thereby measuring,in use, the changes of phase shift across the associated beam.

15. Apparatus as claimed in claim 14, wherein one or more intermediatetransmitting transducers and one or more intermediate receivingtransducers are provided for each beam whereby each beam can be passedthrough the flowing fluid two or more times.

16. Apparatus as claimed in claim 14, wherein each phase-to-voltageconverter comprises a linear quarter period delay network having aninput connected to said first input thereof, a level detector having aninput connected to the output of the linear half period delay network, ahalf period monostable circuit having an input connected to the outputof the level detector, a gate having a first input connected to theoutput of the half period monostable circuit and a second inputconnected to said second input of the converter, and an integratorhaving an input connected to the output of the gate, the output signalof the integrator being one of said two noise signals.

17. Apparatus as claimed in claim 16, wherein each phase-to-voltageconverter includes a first divider connected between said first inputthereof and the input of the linear half period delay network and asecond divider of the same ratio as the first divider connected betweensaid second input thereof and the second input of said gate.

18. Apparatus as claimed in claim 17, wherein each phase-to-voltageconverter includes first and second gating means each having a firstinput connected to a respective one of said first and second inputs ofthe converter, and a pulse width detector having an input connected tothe output of said gate and an output connected to a second input ofeach of said gating means and to reset terminals of each of saiddividers.

19. Apparatus as claimed in claim 6, wherein said transmittingtransducers are of the type responsive to an electrical input signal andwherein said receiving trans-ducers are of the type producing electricaloutput signals.

20. Apparatus as claimed in claim 1, wherein said transmitting means isof the type such that the radiant energy is ultrasonic.

21. Apparatus as claimed in claim 1, wherein said transmitting means isof the type such that the radiant energy is electromagnetic.

22. Apparatus as claimed in claim 21, wherein said transmitting means isof the type such that the electromagnetic radiant energy is within theoptical band.

23. Apparatus as claimed in claim 1, including means for inducingturbulence in the flowing fluid, said means being located upstream ofboth of the beams.

24. A method of measuring the rate of flow of a fluid, comprising thesteps of transmitting a beam of radiant energy into and across the fluidflow at each of two positions spaced apart in the direction of flow ofthe fluid, receiving the two beams after passage across the flow andproducing, for each beam, a beam-representative carrier signal modulatedby a noise signal caused by disturbances in the fluid flow profile,detecting said noise signals, cross-correlating the two detected noisesignals, and determining the value of correlation delay producingmaximum correlation between the two noise signals.

25. A method as claimed in claim 24, wherein the two beams aretransmitted in a common plane which also includes the direction of fluidflow.

26. A method as claimed in claim 24, wherein the two beams aretransmitted in respective different planeseach of which includes thedirection of fluid flow.

27. A method claimed in claim 26, wherein said two planes areorthogonal.

28. A method as claimed in claim 24, wherein noise amplitude modulationon the two beams is detected to provide said two noise signals.

29. A method as claimed in claim 24, wherein noise frequency modulationon the two beams is detected to provide said two noise signals.

30. A method as claimed in claim 24, wherein noise phase modulation onthe two beams is detected to provide said two noise signals.

31. A method as claimed in claim 24, wherein said radiant energy isultrasonic.

32. A method as claimed in claim 24, wherein said radiant energy iselectromagnetic.

33. A method as claimed in claim 32, wherein said electromagneticradiant energy is within the optical band.

34. A method as claimed in claim 24, having the further step of inducingturbulence in the flowing fluid upstream of both said beams.

1. Apparatus for measuring the rate of flow of a fluid, comprising meansfor transmitting a beam of radiant energy, into and across the fluidflow at each of two positions spaced apart in the direction of flow ofthe fluid, respective receiving means associated with each beam and eachpositioned to receive the associated beam after passage across the flow,each receiving means producing a beam-representative carrier signalmodulated by a noise signal caused by disturbances in the fluid flowprofile, a respective detector connected to each receiving means fordetecting said noise signals, and a signal correlator forcrosscorrelating the two detected noise signals whereby the value ofcorrelation delay producing maximum correlation between said two noisesignals can be ascertained.
 2. Apparatus as claimed in claim 1, whereinthe signal correlator includes means for determining the value ofcorrelation delay producing maximum correlation between said two noisesignals.
 3. Apparatus as claimed in claim 1, wherein said transmittingmeans is arranged to transmit the two beams in a common plane which alsoincludes the direction of fluid flow.
 4. Apparatus as claimed in claim1, wherein the transmitting means is arranged to transmit the two beamsin respective different planes each of which includes the direction offluid flow.
 5. Apparatus as claimed in claim 4, where said two planesare orthogonal.
 6. Apparatus as claimed in claim 1, wherein saidtransmitting means includes a respective transmitting transducer foreach beam and wherein said receiving and detecting means includes arespective receiving transducer for each beam.
 7. Apparatus as claimedin claim 6, wherein an oscillator is connected to the input of eachtransmitting transducer.
 8. Apparatus as claimeD in claim 6, wherein theoutput of each receiving transducer is connected by a feedback loop tothe input of the associated transmitting transducer, each feedback loophaving sufficient gain to cause it to self-oscillate so as to producethe associated beam.
 9. Apparatus as claimed in claim 8, wherein theparameters of the components of each feedback loop are selected so thatit oscillates at a frequency which is near or equal to the resonantfrequencies of the associated transmitting transducer and receivingtransducer.
 10. Apparatus as claimed in claim 6, wherein the output ofthe receiving transducer associated with a first of the beams isconnected to the input of the transmitting transducer associated withthe second of the beams and the output of the receiving transducerassociated with the second beam is connected to the transmittingtransducer associated with the first beam, whereby both beams areincluded in a single feedback loop having sufficient gain to cause it toself-oscillate to produce said beams.
 11. Apparatus as claimed in claim10, wherein the two transmitting transducers are mutually matched andwherein the two receiving transducers are mismatched both mutually andrelative to the transmitting transducers.
 12. Apparatus as claimed inclaim 6, wherein said receiving and detecting means includes arespective AM detector connected to the output of each receivingtransducer for detecting noise amplitude modulation on the beams toprovide said two noise signals.
 13. Apparatus as claimed in claim 6,wherein said receiving and detecting means includes a respective FMdetector connected to the output of each receiving transducer fordetecting noise frequency modulation on the beams to provide said twonoise signals.
 14. Apparatus as claimed in claim 6, wherein saidreceiving and detecting means includes, for each beam, a respectivephase-to-voltage converter having a first input connected to the inputof the associated transmitting trans-ducer and a second input connectedto the output of the associated receiving transducer, each converterthereby measuring, in use, the changes of phase shift across theassociated beam.
 15. Apparatus as claimed in claim 14, wherein one ormore intermediate transmitting transducers and one or more intermediatereceiving transducers are provided for each beam whereby each beam canbe passed through the flowing fluid two or more times.
 16. Apparatus asclaimed in claim 14, wherein each phase-to-voltage converter comprises alinear quarter period delay network having an input connected to saidfirst input thereof, a level detector having an input connected to theoutput of the linear half period delay network, a half period monostablecircuit having an input connected to the output of the level detector, agate having a first input connected to the output of the half periodmonostable circuit and a second input connected to said second input ofthe converter, and an integrator having an input connected to the outputof the gate, the output signal of the integrator being one of said twonoise signals.
 17. Apparatus as claimed in claim 16, wherein eachphase-to-voltage converter includes a first divider connected betweensaid first input thereof and the input of the linear half period delaynetwork and a second divider of the same ratio as the first dividerconnected between said second input thereof and the second input of saidgate.
 18. Apparatus as claimed in claim 17, wherein eachphase-to-voltage converter includes first and second gating means eachhaving a first input connected to a respective one of said first andsecond inputs of the converter, and a pulse width detector having aninput connected to the output of said gate and an output connected to asecond input of each of said gating means and to reset terminals of eachof said dividers.
 19. Apparatus as claimed in claim 6, wherein saidtransmitting transducers are of the type responsive to an electricalinput signal and wherein said rEceiving trans-ducers are of the typeproducing electrical output signals.
 20. Apparatus as claimed in claim1, wherein said transmitting means is of the type such that the radiantenergy is ultrasonic.
 21. Apparatus as claimed in claim 1, wherein saidtransmitting means is of the type such that the radiant energy iselectromagnetic.
 22. Apparatus as claimed in claim 21, wherein saidtransmitting means is of the type such that the electromagnetic radiantenergy is within the optical band.
 23. Apparatus as claimed in claim 1,including means for inducing turbulence in the flowing fluid, said meansbeing located upstream of both of the beams.
 24. A method of measuringthe rate of flow of a fluid, comprising the steps of transmitting a beamof radiant energy into and across the fluid flow at each of twopositions spaced apart in the direction of flow of the fluid, receivingthe two beams after passage across the flow and producing, for eachbeam, a beam-representative carrier signal modulated by a noise signalcaused by disturbances in the fluid flow profile, detecting said noisesignals, cross-correlating the two detected noise signals, anddetermining the value of correlation delay producing maximum correlationbetween the two noise signals.
 25. A method as claimed in claim 24,wherein the two beams are transmitted in a common plane which alsoincludes the direction of fluid flow.
 26. A method as claimed in claim24, wherein the two beams are transmitted in respective different planeseach of which includes the direction of fluid flow.
 27. A method claimedin claim 26, wherein said two planes are orthogonal.
 28. A method asclaimed in claim 24, wherein noise amplitude modulation on the two beamsis detected to provide said two noise signals.
 29. A method as claimedin claim 24, wherein noise frequency modulation on the two beams isdetected to provide said two noise signals.
 30. A method as claimed inclaim 24, wherein noise phase modulation on the two beams is detected toprovide said two noise signals.
 31. A method as claimed in claim 24,wherein said radiant energy is ultrasonic.
 32. A method as claimed inclaim 24, wherein said radiant energy is electromagnetic.
 33. A methodas claimed in claim 32, wherein said electromagnetic radiant energy iswithin the optical band.
 34. A method as claimed in claim 24, having thefurther step of inducing turbulence in the flowing fluid upstream ofboth said beams.